Continuous coaters have been known for a number of years and have been widely accepted in industry for achieving rapid, uniform, and economic coating of manufactured articles. Such coaters are now used for coating products with paints and the like, including acrylics, alkyds, asphaltums, oil and water base paints, various varnishes, and vinyls. In some of the coating operations, air can be tolerated in the coating booth or chamber, while in the other applications a solvent-rich atmosphere is maintained.
In coating booths utilizing a solvent-rich atmosphere, the coating material is atomized hydraulically and directed onto a substrate to be coated. By hydraulic atomization, it is meant that the coating material is atomized by means of an orifice through which coating material is pumped at high pressure. When the coating material is sprayed, some of the solvent present in the coating material evaporates upon discharge from the nozzle. Shortly, the solvent vapors expand as a cloud, displacing substantially all of the air in the interior of the coating booth, and provides for a solvent-rich atmosphere therein.
Solvent-rich atmosphere coaters result in a condition within the spray booth where oversprayed material does not tend to solidify on the interior of the chamber walls, or other interior surfaces. This expanding solvent-rich cloud is desirably maintained within the coating booth and prevented from escaping into the work area in which the continuous coater is maintained. Since most continuous coaters have entrance and exit openings in them for the transport of products, special care must be taken to prevent escape of the solvent vapors within the booth. It has been previously known to vent this expanding cloud of vapors into a recovery area adjacent both the entrance and the exit.
Various types of filtering and recovery systems have been incorporated with continuous coaters in the past. For example, U.S. Pat. No. 4,185,975, teaches an embodiment for a coater exhaust which has been well received and widely used in the industry. Similarly, the state of the art of vapor exhaust and recovery systems for continuous coaters is set forth in Nordson Continuous Coater, publication No. 306-18-365, published in 1980 by Nordson Corporation of Amherst, Ohio, the assignee of the instant invention. These prior art structures all teach a vapor recovery system which communicates with the spray booth at the entrance and exit thereof, drafting the expanding cloud of vapor into the system. Most generally, holes or slots in the housing of the entrance and exit provide for such communication.
The known systems typically comprise a plurality of chambers. The first chamber receives the vapor and air through the holes or slots and passes them on to a second chamber where the vapor is subjected to a precipitant spray, causing paint particles and other contaminants in the vapor to be precipitated out of the vapor. This second chamber is typically isolated by sheet metal or the like from the first chamber, such that the precipitant spray does not have an opportunity to pass through the holes or slot and enter into the spray booth or into the area of the conveyor transporting the workpiece to the spray booth.
Subsequent to the second chamber, there has typicaly been provided a plenum chamber to receive the precipitated paint particles and contaminants, allowing them to drop out of the plenum chamber and into a recovery tank positioned therebelow. In order to allow very small particles to settle, the plenum chamber must be of a large size; particles of 3 microns settling in an eleven foot diameter chamber. Indeed, the plenum chamber acts as a settling area of the precipitated paint and contaminants.
Above the plenum chamber is an exhaust stack, generally having a fan positioned at the top thereof to draw the air and vapor, which has now been scrubbed, upward for dissipation into the atmosphere. Positioned at the top of the stack, just before the exhaust fan, prior art has taught the positioning of a precipitation baffle, providing means for condensation of the precipitated vapor with the resulting condensed liquid dropping from the precipitation baffle down the exhaust stack and into the recovery tank. Accordingly, particles are first allowed to settle out of the precipitated vapor in the settling area defined by the plenum chamber, with further settling being achieved by a precipitation or moisture separation baffle positioned at the top of the exhaust stack.
Previously, the vapor and air have been drawn through the holes or slots in the entrance and exit by means of the draft generated by the precipitant spray achieved in the second chamber. An exhaust fan was used in the exhaust stack only to aid this draft generation, it being desired that the draft be a gentle one such that the plenum chamber would be sufficiently quiescent to allow the precipitated matter to drop out into the settling tank. Accordingly, it was difficult to obtain designs having sufficient air flow to maintain the cloud within the spray booth while having a plenum chamber sufficiently quiescent to allow for the settling of small particles.
In substance, the prior art has taught the need of three separate chambers. The first, or inlet chamber, was configured to prevent the precipitant spray from reaching the coating booth or the entrance or exit through which the product passed. The second chamber was the precipitant spray chamber in which the air and vapor were subjected to the spray of a suitable precipitant such as water. The third chamber was that of the plenum, being of sufficiently large geometric size to accommodate the settling of small particles. Below the plenum chamber was a settling tank to which the precipitant or water returned along with the settled-out paint or other contaminants. Above the plenum chamber was the exhaust stack, being characterized by a precipitant or moisture separation baffle for final condensation of particulate matter. It will be appreciated that these baffles provide a tortuous path for air to escape, such path accommodating condensation. Finally, in the settling tank, the paint and other contaminants which rise to the top are filtered and recycled for use in the spray booth.
An inherent problem has been apparent with these known structures. While they have operated efficiently, their sheer size, obstructing otherwise usable space, has been undesirable. Further, they have been of a complex nature, requiring intricate sheet metal work to separate the three chambers from each other, while obtaining the necessary flow path to obtain the desired precipitation and separation. Additionally, the known systems have required frequent maintenance, particularly cleaning of the first or inlet chamber, which chamber is subjected to a combination of ambient air and vapor containing paint or coating particles. Since the air entering the first chamber is no longer solvent-rich, the paint or coating has a tendency to adhere and bond to the walls of that chamber. Additionally, the precipitation baffle in the exhaust stack is subject to frequent clogging and plugging. Accordingly, both the first chamber and the baffle require frequent and routine cleaning to maintain the operability of the recovery system.